Any automaker can stuff a big V8 into a tiny convertible, but Mercedes-Benz chose the efficient route with their performance-focused AMG spinoff. The new 2012 SLK55 AMG Roadster is the most powerful SLK yet, despite a cylinder deactivation system that results in 30 percent better fuel economy. Other efficiency measures include direct injection and a start/stop function that automatically shuts the engine down at red lights.

The 2012 Mercedes-Benz SLK55 AMG is the only car in its class with an eight-cylinder engine, which is essentially a re-worked, naturally aspirated version of the turbocharged engine found in bigger AMG cars. In contrast to the turbocharged 4-cylinder SLK250 and V6-powered SLK350, the souped-up SLK55 packs a tighter suspension setup, beefier brakes, sportier interior, and more aggressive bodywork.

Climb Inside: Snug, Driver-Focused Cockpit

Slide into the SLK55 AMG's low-slung interior, and you're surrounded by a blocky yet well-finished layout. In comparison to the sleek curves found in the cockpits of its competitors, the SLK's dashboard panel meets the doors with relatively square intersections. But the cabin's pleasingly finished surfaces reflect a thoughtful attention to detail. All SLK55s are appointed with a tidy analog IWC clock found front and center on the dash, and a flat-bottomed, 3-spoke steering wheel complements the top-stitched surfaces throughout. Four large air conditioning vents mimic jet engine intakes, visually connecting with several other aluminum bits sprinkled in the cozy cockpit.

The 8-way adjustable seats are more supportive than standard issue SLKs, and various AMG badges serve as not-so-subtle reminders that you're driving a tuned version of the famously style-conscious car. The 2012 model features more standard safety equipment including attention assist, LED taillamps, adaptive brakes, and a more comprehensive array of airbags. Also standard is a new fiber-reinforced rollbar that automatically deploys in case of an accident.

The SLK55 also offers the Magic Sky Control option, a new-for-2012 feature which enables the glass roof to transform from opaque to transparent at the touch of a button. Other options include a panoramic roof, multimedia package which includes a navigation system with a 10-gigabyte hard drive, and headlamps with active curve illumination. An AMG handling package adds a sportier steering wheel, stiffer suspension, 18-inch wheels, a differential lock, and a higher top speed of 174 mph.

On the Road: Zippy, Swift, and Grippy

While lesser-powered SLK models boast strong acceleration and buttoned-down handling, the SLK55 is a different beast altogether. The AMG version's V8 produces 415 horsepower and 398 lb-ft of torque, and connects to a Speedshift Plus 7-speed that operates in efficiency, sport, or manual mode. In its most conservative setting, the transmission enables the engine's stop/start function and cylinder deactivation-though the latter feature is not available during top-down driving. Apparently, Mercedes-Benz engineers assume enthusiasts prefer the full performance monty while driving al fresco.

Less than 20 seconds are required for the SLK's hardtop to fold into the car's body, but the more impressive spec is the scant 4.5 seconds it takes to sprint to 60 mph. Acceleration in the SLK55 is swift and smooth, thanks to the engine's gutsy, flexible powerband and the seven-speed transmission's well-spaced ratios. A modulated exhaust note ensures that engine sounds aren't intrusive during mellow driving; under low loads and engine speeds below 2,000 rpm, flaps are closed for quieter cruising. But under heavier throttle, they open incrementally to enable a louder exhaust note.

During our test drive on the mountainous Skyline Boulevard along the Central California coast, the SLK55's engine offered just the right amount of bark without producing an annoying drone inside the cabin. Top-down turbulence is dramatically reduced thanks to a removable mesh screen between the roll hoops, and our drive was also made comfortable by the SLK's seat heaters and Airscarf system, which blows hot air from the headrests. Our tester included the optional handling package, which yielded an unsurprisingly stiff but responsive ride.

In the sportiest automatic transmission mode, the 7-speed shifts rapidly at higher rpms, though the computer is a bit conservative at lower engine speeds; several attempts at aggressive off-the-line acceleration required an extra downshift, surprising for the performance-oriented sports car. We had less luck with manual mode, whose shifts were accompanied with a noticeable delay. Despite our attempts to anticipate the 7,200 rpm redline, if we didn't shift several hundred rpm prior to the rev limiter we were met with a pesky pause. As tempting as it may be to command the transmission via the steering wheel-mounted paddle shifters, it's best to leave the gearbox in sport mode, which offers crisp, quick shifts.

Our time spent testing the SLK55 also saw a brief stint with the top up, which enabled us to observe the cylinder deactivation function in action. The process occurs when revs are between 800 and 3,600 rpm in the most efficient mode, and the change takes as little as 30 milliseconds. The switch from four to eight-cylinder mode is subtle, and almost feels like an incremental surge of power; were it not for a dashboard indication, we might not have even noticed. Up to 170 lb-ft of torque are available when the engine runs on four cylinders, allowing enough power for cruising.

At the end of the day, the SLK55's mission is pinpointed by AMG head Ola Kallenius, who puts the Mercedes into the context of more enthusiast-oriented options like the Porsche Boxster: "It's mainly a lifestyle car," he says of the littlest AMG, an admission that doesn't diminish the roadster's impressive capabilities, but rather emphasizes its place in the German sub-brand's expanding repertoire of performance cars.

This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.